docs: passkeys with user import disabled EA

[hazel-nut]

Description

EA docs for passkey support for custom dbs with import mode disabled, plus a vigorous content update for the passkey landing page + configure passkeys how-to.

References

DOCS-5355

Testing

the new content is in the labeled section in the prerequisites. i'm interested in opinions on the updated content and formatting of all of the revised content.

Checklist

• I've read and followed CONTRIBUTING.md.
• I've tested the site build for this change locally.
• I've made appropriate docs updates for any code or config changes.
• I've coordinated with the Product Docs and/or Docs Management team about non-trivial changes.

[github-actions]

Summary

Status	Count
🔍 Total	20
✅ Successful	1
⏳ Timeouts	0
🔀 Redirected	1
👻 Excluded	18
❓ Unknown	0
🚫 Errors	0
⛔ Unsupported	0

Redirects per input

Redirects in main/docs/authenticate/database-connections/passkeys.mdx

• [200] https://fidoalliance.org/passkeys | Redirect: Followed 1 redirect resolving to the final status of: OK. Redirects: https://fidoalliance.org/passkeys --> https://fidoalliance.org/passkeys/
  Full Github Actions output